Structural bearings are intended to be interposed between a support and a member such as a slab or beam supported thereby. The structural bearing absorbs relative movement between the support and the beam or slab. Such movement may be caused by, for example, temperature changes, curing shrinkage of concrete, or settling of foundations. The movement may be horizontal displacement of the slab or beam and/or rotational movement of the slab or beam about a horizontal axis.
A first known type of bridge bearing is in the form of a monolithic block consisting of a stack of parallel metal plates, which in use of the bearing are horizontal, embedded in rubber. Layers of rubber separate each two adjacent plates and cover the top plate and the bottom plate. Also rubber completely covers the edges of the plates. Thus there are no exposed surface areas of the metal plates and the metal plates are protected against rusting or other corrosion. In use of the bearing the layers of rubber deform to absorb relative movement between the support and the beam or slab and the metal plates resist excessive laterally outwards or horizontal deformation of the rubber. The bearing is manufactured by making a stack of the metal plates and unvulcanized rubber sheets, the rubber sheets being disposed between each pair of adjacent metal plates and below the bottom plate and above the top plate. The stack of metal plates and rubber sheets is then subjected to pressure (applied to the top and bottom of the stack) and to heat to cause the rubber to vulcanize and to cause the rubber to form an integral body containing the metal sheets. Disadvantages of this vulcanization process are that the layers of rubber between the metal plates tend to be of variable uncontrolled thicknesses and it is difficult to ensure that the rubber at the interior of the bearing is satisfactorily vulcanized and the rubber adjacent the exterior of the bearing is not overvulcanized. A further disadvantage is that the vulcanization process has to be carried out slowly to control, as far as possible, the degree of vulcanization throughout the bearing. Consequently the rate of production of the bearing is slow and, in view of the capital cost of the necessary vulcanization equipment, costly. Another disadvantage is that the bridge bearing has to be made as a single unit of the desired size.
The aforementioned disadvantages are overcome or mitigated by a second type of bridge bearing, such as disclosed in British patent specification No. 1,192,744 (originally in the name of Silent Channel Products Limited). This bridge bearing comprises a stack of modular elements, namely an upper modular element, one or more intermediate modular elements and a lower modular element. The or each intermediate element has a layer of rubber adhered to and interposed between two metal plates. The upper element has a layer of rubber on top of and adhered to a metal plate and the lower element similarly has a layer of rubber below and adhered to a metal plate. The plates are provided with holes in which are located circular members such as rings or discs which key together the adjacent metal plates of adjacent elements, each circular member being located in corresponding holes in both of the plates. To prevent relative rotation of each two keyed together plates, it is necessary that at least two of the circular members are used to key together the plates. The layers of rubber overlap the metal plates and extend around and are adhered to the edges of the metal plates but the opposed faces of the metal plates of adjacent elements are free of rubber. Since the edges of the plates are covered by rubber, the metal plates are effectively encased by rubber and protected against corrosion. The elements after manufacture can be assembled into a bridge bearing of the desired height by using a selected number of intermediate elements. However, one disadvantage of the bearing is that moisture can penetrate between adjacent elements and cause corrosion of the metal plates at their surfaces not covered by rubber. Another disadvantage is that when the rubber has a tendency to break away from the edges of the plates when the bearing is under load and the rubber layers are being compressed and deformed laterally and outwardly. Yet another disadvantage is that the exposed metal surfaces of the elements tend to corrode on storage prior to assembly to form the bridge bearing.
The disadvantages referred to above are overcome or mitigated by a third type of bridge bearing disclosed in British patent specification No. 2,054,092A (Dixon International Limited). In this type of bridge bearing, both the upper and lower surfaces and the edge of each metal plate are covered by rubber.
It is normal with bridge bearings of the second and third types to adhere the assembled elements together prior to installation in a bridge structure. The purpose of this is to facilitate handling of the bearing and to prevent the bearing coming apart and the keying members, which are essential, being lost or not replaced in the bearing.
The intermediate elements of the second types of bridge bearing are manufactured by locating the lower metal plate of the element on the bottom mould plate of a press, placing a plurality of sheets of rubber on the lower metal plate, and locating the upper metal plate on the top mould plate of the press, the upper metal plate being held against the top mould plate by magnets. Both the lower and upper metal plates are accurately located by pins on the bottom and top mould plates, respectively mounted on the upper and lower platens of the press, the pins engaging in openings in the plate. The press is then operated to compress the sheets of rubber between the plates and to heat and vulcanize the rubber.
The intermediate element of the third type of bridge bearing is manufactured similarly to the intermediate element of the second type of bridge bearing but, in addition, sheets of rubber are placed between the bottom mould plate and the lower metal plate and between the upper metal plate and the top mould plate.
The upper and lower elements are also manufactured similarly in a press, but only one metal plate is used in each element.
Although in the manufacture of the intermediate elements of both the second and third types of bridge bearings the upper and lower plates can be accurately located, the locating of the upper plate tends to be time consuming. Moreover if the upper plate is curved or otherwise deformed from a planar state, as not infrequently happens, (due to e.g. metal surface treatments, such as shot-blasting, for the purpose of preparing the metal surface to achieve good mechanical bonding with the rubber) the plate cannot be held securely to the upper mould plate by the magnets and may become displaced from its desired position.
Moreover, with both the second and third types of bridge bearing it is necessary to manufacture the upper and lower elements (which may be identical) in addition to the intermediate elements.